1. Field of the Invention
This invention relates to a web position detecting method for a device which is adapted to convey and/or dry a continuously supplied elongated belt-shaped support (hereinafter referred to as "a web") under low tension in a non-contact mode while floating the web with air streams jetted from a number of air jetting holes.
2. Background of the Invention
Heretofore, in conveying and/or drying a paper or film web, a web position control device is used in order to eliminate the difficulty that the web is creased, dried irregularly or broken because the web runs zigzag or shifts to a side and the resultant product is low in quality. If a web is detected to be displaced from its desired position, manual or automatic equipment realigns the web by shifting or tilting rollers used for conveying the web.
For the web position control, it is essential to accurately detect the web position. For this purpose, a variety of web position detecting methods have been employed in the art. One web position detecting method uses a special air nozzle. Another web position detecting method uses an image sensor or television camera. A third web position detecting method uses an optical sensor is used. A fourth web position detecting method employs ultrasonic waves.
However, in the case of photo-sensitive materials such as photographing films, the detecting method in which optical sensors are used, or the detecting method in which an image sensor or television camera is employed is not practical because the photo-sensitive materials are sensitive to light and the necessary equipment is intricate. In the case of the detecting method in which ultrasonic waves are employed, the place where it can be used is limited, and the detection accuracy is liable to be low. Accordingly, the web position detecting method in which, as shown in FIG. 1, a special air nozzle 5 is used is extensively employed in the art. In this detecting method, the air nozzle 5 is arranged on one side of a web 4 and an air receiving inlet 6 for receiving the air stream jetted from the air nozzle 5 is disposed on the other side of the web 4 in such a manner that it confronts the air nozzle 5. As a result, the position of the web 4 is detected from the variation in pressure of the air stream received by the air receiving inlet 6.
However, in the case where a web 4 is conveyed and/or dried while being floated by the air stream, i.e., it is conveyed and/or dried in a so-called "non-contact mode". The pressure of the air stream jetted from air jetting hole, being high, greatly disturbs the detecting air jetted from the special detecting air nozzle 5. As a result, sometimes it is impossible to accurately detect the position of the web 4. Furthermore, the detecting method in which the special air nozzle is employed suffers from the difficulty that, when the side edge portions of the web are curled, the side edges portions may touch the nozzle to provide an unacceptable product.
A web position detecting method which is effective in eliminating the above-described difficulties accompanying the conventional detecting method has been disclosed in Japanese Patent Application (OPI) No. 244806/1985. (the term "OPI" as used herein meaning an "unexamined published application"). In the web position detecting method thus disclosed, the back pressure of the web is utilized. However, the detecting method is still disadvantageous in the following points. In this method, the back pressure of a web floated depends considerably on the width and tension of the web. For instance, if the tension is small, then the back pressure is low. Therefore it is difficult to accurately detect the position of the web. If the width of the web changes, the back pressure also changes. In addition, employing the position detection signal obtained in the detecting method as an input signal to the web position control device as it is cannot provide an ideal (web position with back pressure) control characteristic.